The present invention relates to a heterojunction bipolar transistor (hereinafter referred to as HBT) capable of rapidly operating. More particularly the present invention relates to a wide band gap HBT.
To make a conventional homojunction transistor operate rapidly, it is necessary to form a thin base region. However, the thin base region undesirably causes punching through. On the other hand, if the concentration of the base region is made high to prevent punching through, the gain of the transistor is lowered. Thus, there is a limit in realizing both high gain and rapid operation in conventional transistors.
In view of the above-mentioned circumstances, earnest studies on a Si(silicon)-HBT have been made to realize rapid operation in silicon bipolar transistors. As the Si-HBT which have been studied, there are wide-gap-emitter-type ones, wherein a material such as silicon carbide having a wider band gap than that of a silicon is applied to an emitter region, and narrow-gap-base-type ones, wherein a material such as mixed crystal of the silicon and a Ge having a narrower band gap than that of the silicon is applied to a base region.
In a conventional wide band gap HBT, a silicon semiconductor crystal is applied to a collector region and the base region, and 3c-SiC (3c-silicon carbide or .beta.-SiC), which has a wider band gap than that of the silicon, is applied to the emitter region so that a emitter efficiency or injection efficiency increases and a base resistance decreases. Thus the conventional wide band gap HBT is used as an element for rapid operation and high power. These are disclosed in Japanese Unexamined Patent Publication No. 216364/1987.
However, the conventional wide band gap HBT, wherein the same semiconductor material (silicon) as that of the base region is applied to the collector region and an injection of positive holes that the positive holes move from the base to the collector are raised, has a problem that a large amount of positive holes accumulate in the collector region due to a comparatively thick collector region so that diode operation becomes tardy or late.
To overcome the above-mentioned problem, a HBT rapid operation has been disclosed in Japanese Patent Application No. 251918/1991, which corresponds to U.S. Pat. No. 5,247,192 wherein the collector region is also formed of a 3c-SiC crystal layer. More specifically, the HBT includes, as shown in FIG. 9, a 3c-SiC crystal layer 22 (collector region), a silicon crystal layer 23 (base region) and another 3c-SiC crystal layer 24 (emitter region) which are formed in this order in an opening defined in insulating films 25 and 27 covering a semiconductor substrate 21. A polysilicon film 26 is formed in advance between the insulating films 25, 27 for providing a connection between the base region 23 and a base electrode 28. The reason why such a constitution is developed is that the SiC crystal layer is hard to be etched with etching agent and hence severity in processing such as ion-milling is required even for removing unnecessary portions thereof. With a common constitution, upon processing the emitter region 24 the base region 23 is also processed undesirably, resulting in degradation in device characteristics.
To avoid such disadvantage attributable to the common constitution, the polysilicon film 26 is formed in advance to provide the connection between the base region 23 and the base electrode 28. In forming the base region 23, side faces thereof have to be coincident with the polysilicon film 26. For this reason, the thickness of the base region 23 needs to have somewhat of allowance. Thus the base region 23 cannot have such a thickness as to be required in a device characteristics.
In addition, there is a limitation to a certain degree in controlling a thickness of the film of a semiconductor crystal layer epitaxially grown. As a result of the above-mentioned fact that the base region 23 cannot be made sufficiently thin, there arises a problem that the desired device is prevented from rapidly operating.